No, she is being unreasonable. Has she even met the ex-girlfriend? Does she give any specific reason for hating her? Even if she does hate her, I say you are free to name your future children whatever you want. She can't tell you what to do. Besides, people rarely go by their middle names, and they are never mentioned unless someone asks. The odds of your daughter actually being called
Claire are practically zilch, so tell your sister to chill out.